Method for gettering transition metal impurities in silicon crystal

ABSTRACT

Disclosed is a method for gettering a transition metal impurity diffused in a silicon crystal at ultra high-speeds to form deep impurity levels therein. The method comprises codoping two kinds of impurities: oxygen and carbon, into silicon, and thermally annealing the impurity-doped silicon to precipitate an impurity complex of an atom of the transition metal impurity, the C and the O, in the silicon crystal, so that the transition metal impurity is confined in the silicon crystal to prevent the ultra high-speed diffusion of the transition metal impurity and electrically deactivate deep impurity levels to be induced by the transition metal impurity. The present invention makes it possible to produce a silicon semiconductor device free of adverse affects from a transition metal impurity, such as Co, Ni or Cu, mixed in a silicon crystal during a process of forming the silicon single crystal, or such as Cu mixed in a silicon wafer during a process of printing a Cu wiring, which has not been able to be completely eliminated from the silicon crystal through conventional techniques.

TECHNICAL FIELD

The present invention relates to a method for deactivating a transitionmetal impurity, such as Co, Ni or Cu, which is released from rowmaterials during a process of forming a silicon single crystal and mixedin the crystal as a solid solution, or such as Cu which is mixed in asilicon wafer during a process of printing a Cu wiring, to produce asilicon semiconductor device free of deep impurity levels.

BACKGROUND ART

Based on high-density integration using nano-processing techniques,silicon semiconductor devices serve as the backbone of currentinformation-driven society. In connection with the existing need forhigher operation speed and higher-density integration in siliconsemiconductor devices, the contact resistance between wirings becomes acritical factor dominating the operational limit of these devices.

Heretofore, an aluminum thin wire has been used as a wiring material forhighly integrated silicon semiconductor devices. However, a thinner wirerequired for the high-density integration and ultra-miniaturization ofsilicon semiconductor devices inevitably has higher resistivity andcontact resistance, and resulting increased heat generation causesdeterioration in durability of the devices which hinders higher-densityintegration. In this context, a technique of reducing the resistanceusing a copper (Cu) thin wire has been developed and actually used in apart of CPUs.

During a semiconductor forming process and a wiring printing processusing lithography, Cu atoms are mixed in a silicon device throughdiffusion to form a deep impurity level in the bandgap of a siliconcrystal. Moreover, the Cu atoms are incorporated in the silicon crystalthrough ultra high-speed diffusion to form deep impurity levels all overthe silicon crystal, which are likely to serve as a carrier killer orcause dielectric breakdown. Consequently, in the existing circumstances,the devices using Cu thin wires have poor process yield.

While a transition metal impurity, particularly Co, Ni or Cu, which isreleased from raw materials during the formation of asilicon-single-crystal through a Czochralski crystal pulling process orthe like and mixed in the crystal as a solid solution, is insignificantif the device has a relatively large size, even a small amount oftransition metal impurity residing in the device has a great impact onthe quality and process yield of the device in the present circumstanceswhere the device is ultra-miniaturized in conjunction with the need forhigh densification.

In view of these situations, there has been employed a method, so-calledgettering, for eliminating a transition metal impurity which iscontained in a wafer subject to device processing and likely to serve asa carrier killer, or for confining the transition metal impurity at aposition away from a surface for use in device processing to immobilizeit during heat treatment or device processing (for example, thefollowing Patent Publications 1, 2 and 3)

-   -   Patent Publication 1: Japanese Patent Laid-Open Publication No.        10-303430    -   Patent Publication 2: Japanese Patent Laid-Open Publication No.        2001-250957    -   Patent Publication 3: Japanese Patent Laid-Open Publication No.        2001-274405

However, these conventional techniques have difficulties in producingthe device while completely eliminating any transition metal impuritydefused at ultra high-speeds to form deep impurity levels. Thus, in theproduction process of silicon semiconductor devices, it is an essentialfactor to solve this problem.

DISCLOSURE OF INVENTION

The present invention is fundamentally directed to a method forgettering a transition metal impurity diffused in a silicon wafer atultra high-speeds to form deep impurity levels therein, particularly Co,Ni or Cu which is diffused at ultra high-speeds under a roomtemperature. In this method, two kinds of impurities consisting ofoxygen (O) and carbon (C) are codoped into silicon, and then theimpurity-doped silicon is thermally annealed to form an impurity complexcomprising the C, the O and the transition metal impurity, at a specificatomic position in the silicon crystal, so as to produce a siliconsemiconductor device free of adverse affects from the transition metalimpurity.

A chemical bonding energy in the impurity complex formed in this mannercan confine the transition metal impurity in the impurity complex, andelectrically deactivate deep impurity levels to be induced by thetransition metal impurity. Thus, even if a transition metal impurity,such as Co, Ni or Cu, mixed during a silicon-single-crystal formingprocess, or Cu mixed during a Cu-wiring printing process, exists in asilicon crystal, a silicon semiconductor device free of deep impuritylevels in the bandgap of the silicon crystal can be produced.

More specifically, the present invention provides a method for getteringa transition metal impurity diffused in a silicon crystal at ultrahigh-speeds to form deep impurity levels therein. This method comprisesthe steps of codoping two kinds of impurities consisting of oxygen (O)and carbon (C), into silicon, and thermally annealing the impurity-dopedsilicon to precipitate an impurity complex comprising an atom of thetransition metal impurity, the C and the O, in the silicon crystal, sothat the transition metal impurity is confined in the silicon crystal toprevent the ultra high-speed diffusion of the transition metal impurityand electrically deactivate deep impurity levels to be induced by thetransition metal impurity.

In the above method of the present invention, the transition metalimpurity may be at least one selected from the group consisting of Co,Ni and Cu which are released from a raw material during a process offorming a silicon single crystal and mixed in the silicon crystal, andCu which is mixed in a silicon wafer during a process of printing a Cuwiring.

The codoping step may include codoping oxygen (0) in a natural mannerand carbon (C) in an artificial manner, or both oxygen (O) and carbon(C) in an artificial manner, into a silicon melt during a silicon singlecrystal growth through a Czochralski crystal pulling process.

Alternatively, the codoping step may include ion-injecting an oxygen ionand a carbon ion into a silicon wafer to codope both oxygen (O) andcarbon (C) in an artificial manner, into the silicon wafer.

A transition metal diffused at ultra high-speeds through an interstitialposition in a silicon crystal, particularly Co, Ni or Cu impurity, formsdeep impurity levels in the bandgap of the crystal, to capture carriersfrom an acceptor and/or donor of p-type and n-type silicon crystals soas to cause significant deterioration in device functions.

As one example, a sample in which Cu is diffused in a wafer (1 Ωcm)formed of a low-resistance n-type silicon single crystal was prepared bydoping Cu into the wafer through an ion injection process to form deepimpurity levels in the bandgap so as to provide a high resistance (10KΩcm) to the wafer. As seen from the measurement result of diffusioncoefficients of Cu and Ni in FIG. 1, Cu and Ni are diffused at ultrahigh speeds having a greater digit number of 10 or more, as compare tothat of a Si atom in the silicon crystal or a p-donor impurity in thesilicon crystal. By way of comparison, FIG. 1 also shows the temperaturedependences of respective diffusion coefficients of the Si atom in thesilicon crystal and the donor impurity in the silicon crystal.

Through the above data, it was verified that Cu doped in a siliconsingle crystal forms deep impurity levels in the bandgap while beingdiffused at ultra high-speeds. Further, it was experimentally provedthat the diffusion barrier of a Cu impurity in a silicon crystal is inan extremely shallow level of 0.18 to 0.35 eV, and the Cu impurity canbe diffused even at a room temperature.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the temperature dependences of respectivediffusion coefficients of Ni and Cu in a silicon crystal.

FIG. 2 is a schematic diagram showing the structure of a C—O impuritycomplex in a silicon crystal, which is formed through a Czochralskicrystal growth process.

FIG. 3 is a schematic diagram showing the structure of a C—O impuritycomplex in a silicon crystal, which is experimentally determined throughan extended x-ray absorption fine structure (EXAFS) spectroscopy.

FIG. 4 is an explanatory diagram showing a process in which a deepimpurity level (a) of a Cu impurity in a silicon crystal is vanishedwhile splitting into a bonding state in the valence band and anantibonding state in the conduction band, in response to the formationof a Cu—O—C impurity complex, and changed into a Cu—O—C impurity level.

FIG. 5 is a graph showing the temperature dependences of respectivediffusion coefficients of Ni and Cu in a silicon crystal, after codopinga C atom and an O atom into the silicon crystal, and then thermallyannealing the imparity-doped silicon crystal.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is directed to a method for gettering a transitionmetal impurity in a silicon crystal, comprising codoping two kinds ofimpurities consisting of oxygen (O) and carbon (C), into silicon, andthen thermally annealing the impurity-doped silicon.

The codoping may be achieved by introducing oxygen or carbon into asilicon melt during the course of forming a silicon crystal through aCzochralski crystal pulling process to be performed in advance of thepreparation of a silicon wafer. While oxygen is generally introducedfrom surrounding air naturally or in a natural manner, its concentrationshould be controlled. Thus, both oxygen (O) and carbon (C) arepreferably codoped artificially or in an artificial manner whilecontrolling the concentration thereof. The oxygen (O) and carbon (C) maybe codoped into a silicon wafer in an artificial manner through an ioninjection process. The codoped oxygen (O) and carbon (C) is set at aconcentration equal to or greater than that of the transition metalimpurity, for example, in the range of about 10¹⁵ to 10¹⁹ cm⁻³.

If a carbon (C) atom is doped in an artificial manner at aSi-substituting position in a silicon crystal, a strain field withlong-range interactions will be formed because the carbon (C) atom hasan atomic radius less than that of a silicon (Si) atom. The oxygen (O)doped into a silicon crystal in a natural manner or in an artificialmanner through a Czochralski crystal growth process, or the oxygen (O)doped into a silicon crystal in an artificial manner through an ioninjection process, gets at an interstitial position of the silicon bond.

Then, the silicon crystal containing the two kinds of doped C and O issubjected to a thermal annealing treatment. For example, the thermalannealing treatment is performed by placing a silicon wafer in anelectric furnace, and heating under a nitrogen gas or argon gasatmosphere at a temperature of 250° C. or more, preferably at atemperature ranging from about 350 to 500° C. for about 10 minutes to 2hours. As shown in FIG. 2, through the thermal annealing treatment, Oatoms getting at the interstitial position of the silicon bond arecollected around a C atom at the Si-substituting position due to thestrain field with long-range interactions arising from the C atom. The Catom is located at the central position of the interstitial bond.

Simultaneously, the strain field with long-range interactions in the Catom allows an atom of the transition metal impurity to be weakly drawnto the C atom, so as to form an impurity complex in combination with theO atoms collected around the C atom through the thermal annealingtreatment, and precipitate the impurity complex consisting of thetransition metal, the C atm and the O atoms, at a specific atomicposition in the silicon crystal. The structure of the impurity complexcontaining the transition metal was experimentally determined through anEXAFS spectroscopy. As a result, it was proved that the impurity complexhas a configuration as shown in FIG. 3. The specific atomic positionherein means an interstitial position which is located in the vicinityof the carbon (C), and allows the transition metal to be strongly bondedwith the oxygen (O) so as to form a compound, as shown in FIG. 3.

Based on a chemical bonding force in the complex formed of thetransition metal impurity and the two kinds of impurities consisting ofthe oxygen (O) and the carbon (C), the transition metal impurity isconfined in the impurity complex. In addition, as illustrated in FIG. 4,according to a strong orbital hybridization of the 3d-orbital of thetransition metal and the p-orbitals of the C atom and the O atoms, adeep impurity level is split into a bonding state (in the valence band)and an antibonding state (in the conduction band), and vanished. Thus,the deep impurity level can be electrically deactivated.

The respective diffusion coefficients of Cu and Ni in this state weremeasured. As shown in FIG. 5, the diffusion coefficients have a reduceddigit number of about 8 to 9, and exhibit almost no diffusion. By way ofcomparison, FIG. 5 also shows the temperature dependences of respectivediffusion coefficients of a Si atom in the silicon crystal and a p-donorimpurity in the silicon crystal.

According to the method of the present invention, in a process ofproducing a silicon semiconductor device, the electrical activity andultra high-speed diffusion of a transition metal impurity can becontrolled by a simple thermal annealing treatment during the deviceproduction process. Thus, it is expected to provide a drastic effect onachieving a high-speed and energy-saving operation, which isadvantageous to silicon semiconductor industries.

In addition, such a production process technology can be used foraccelerating high-speed operation, high-densification and/or energysaving in all devices using a silicon crystal. Thus, this technology hasextremely wide application, and can serve as one basic technical factorindustrially essential for silicon device productions in the future.

EXAMPLE 1

The codoping of oxygen (O) and carbon (C) in a silicon crystal, theanti-diffusion of a Cu impurity through a thermal annealing treatmentand the deactivation of a deep impurity level will be described inconnection with a specific example.

During the course of performing crystal growth through a crystal pullingprocess using a Czochralski crystal pulling apparatus, oxygen (O) andcarbon (C) were codoped into a silicon melt. Through this process, alow-resistance n-type silicon single crystal doped with the oxygen (O)and carbon (C) at a concentration of 8×10¹⁸ cm⁻³ which is equal to orgreater than that of a copper impurity was obtained. A wafer obtainedfrom this single crystal had an electrical resistivity of 1 Ωcm. Inorder to prepare a sample analogous to a wafer containing a copperimpurity mixed therein during a Cu-wiring printing process, Cu was dopedinto the above wafer at a concentration of 4×10¹⁸ cm⁻³ through an ioninjection process. Then, the wafer was placed in an electric furnace,and thermally annealed under an argon atmosphere at each of temperaturesof 100° C., 200° C., 300° C., 350° C., 400° C. and 500° C., for 16minutes.

The electrical resistivity of the wafer after the thermal annealingtreatment was measured. As shown in Table 1, in case of the thermalannealing treatment at a temperature of 350° C. or more, the electricalresistivity of the wafer was not substantially changed from 1 Ωcm. Awafer subjected to no thermal annealing treatment (annealingtemperature—in Table 1) has a resistivity of 8569 Ωcm. In view of thesedata, it was verified that deep impurity levels due to the doped Cu arevanished by the codoping of the oxygen and carbon and the thermalannealing treatment. TABLE 1 annealing temperature — 100 200 300 350 400500 resistivity after 8569 367 58.0 9.60 1.02 0.98 0.67 annealing for 16minutes (Ωcm)

Industrial Applicability

According to the present invention, the electrical activity and ultrahigh-speed diffusion in deep impurity levels of a transition metalimpurity, such as Co, Ni or Cu, can be controlled by a simple treatmentduring a device production process. Thus, it is expected to provide adrastic effect on achieving a high-speed and energy-saving operation,which is advantageous to silicon semiconductor industries. The presentinvention can contribute to provide high-performance siliconsemiconductor devices.

1. A method for gettering a transition metal impurity diffused in asilicon crystal at ultra high-speeds to form deep impurity levelstherein, said method comprising the steps of: codoping two kinds ofimpurities consisting of oxygen (O) and carbon (C), into silicon at aconcentration equal to or greater than that of at least one transitionmetal impurity selected from the group consisting of Co, Ni and Cu whichare released from a raw material during a process of forming a siliconsingle crystal and mixed in said silicon crystal, and Cu which is mixedin a silicon wafer during a process of printing a Cu wiring; andthermally annealing said impurity-doped silicon at a temperature rangingfrom 250° C. to 500° C. to form a transition metal —O—C complexcomprising an atom of said transition metal impurity, said C and said O, so as to precipitate said imparity complex at an interstitial positionin said silicon crystal, whereby said transition metal impurity isconfined in said silicon crystal to prevent the ultra high-speeddiffusion of said transition metal impurity and electrically deactivatedeep impurity levels to be induced by said transition metal impurity. 2.(canceled)
 3. The method as defined in claim 1, wherein said codopingstep includes codoping oxygen (O) in a natural manner and carbon (C) inan artificial manner, or both oxygen (O) and carbon (C) in an artificialmanner, into a silicon melt during a silicon single crystal growththrough a Czochralski crystal pulling process.
 4. The method as definedin claim 1, wherein said codoping step includes ion-injecting an oxygenion and a carbon ion into a silicon wafer to codope both oxygen (O) andcarbon (C) in an artificial manner, into said silicon wafer.